Conventionally, a coordinate input device used to control a computer connected and to write characters, figures, and the like by directly inputting a coordinate position using a pointing device on the screen of a large-scale display is known. As a coordinate input device of this type which adjusts itself in correspondence with a change in screen upon, e.g., zooming, a device that uses a position detection device (an analog device which outputs an output voltage corresponding to the spot position) called a PSD is known.
For example, Japanese Patent Publication No. 7-76902 discloses a device which detects a coordinate position by sensing an image of a beam spot formed by a collimated beam of visible light, and simultaneously transmits/receives control signals by infrared divergent light. Also, Japanese Patent Laid-Open No. 6-274266 discloses a device which detects a coordinate position using a linear CCD sensor and special optical mask. Furthermore, Japanese Patent Laid-Open No. 5-224636 discloses a device which inserts a half mirror into the optical path of a projection lens to form an image on the screen on a PSD, thus detecting the coordinate position.
In recent years, the brightness of the screen of a large-screen display has improved, and such display can be satisfactorily used even in a brightly illuminated environment. Also, since computers have prevailed, demands of such displays are increasing. Especially, in a presentation or meeting using a computer screen, a coordinate input device that can directly operate the screen is very convenient. Especially, a front projection type device is portable, and can be used while changing the screen size in correspondence with the location where it is used.
However, in the coordinate input device described in Japanese Patent Laid-Open No. 5-224636, the wavelength selective half mirror is inserted in the projection optical path, and may adversely influence the image quality of a projected image. To avoid such shortcoming, a high-precision mirror must be used. Also, an insertion space for the mirror is required between the projection lens and a liquid crystal panel. For this reason, a large projection lens size is required to assure a sufficient amount of light, resulting in high cost. Furthermore, a sensor which has a small size and is inexpensive compared to the liquid crystal panel can be used. However, to use such sensor, a reduction optical system is required. In order to assure sufficiently high brightness of an optical system on the sensor side that includes a reduction optical system, a reduction optical system with a large size is required, resulting in an expensive device.